  <!DOCTYPE HTML PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xHTML1/DTD/xHTML1-transitional.dtd">
<HTML XMLns="http://www.w3.org/1999/xHTML" lang="gb2312">
<head>
<title>24png</title>
<meta name="keywords" content="JS,事件冒泡,cancelBubble,stopPropagation" charset=utf-8/>
<!--[if !IE]> -->
  <script src="http://cdn.bootcss.com/jquery/2.2.0/jquery.min.js"></script>
  <!-- <![endif]-->
  <!--[if IE]>
      <script src="http://cdn.bootcss.com/jquery/1.12.0/jquery.js"></script>
  <![endif]-->
<style>
*{
  margin:0;
  padding:0;
}
  body{
    webkit-text-size-adjust: none;

  }
</style>



</head>
<body>

<script>
var a = {n: 1}
var b = a;  
a.x = a = {n: 2}
console.log(a);
console.log(b);
console.log(a.x);//undefine
console.log(b.x)//{n: 2}


</script>
</body>
</HTML>